There are a number of technologies in which a very small aperture is required.
Near field scanning microscopy, for example, is a technique for analyzing objects by means of a light beam which is smaller than the wavelength of the light. To obtain a light source of subwavelength dimensions, light is directed through an aperture formed in a thin opaque screen and having a width less than one-half the wavelength (.lambda.) of the light, frequently substantially smaller than the wavelength (e.g., .lambda./20). The sample to be studied is positioned in the "near field" of the light source, which begins at the aperture and extends outward a distance equal to about one-half of the width of the aperture, wherein the light emerging from the aperture remains collimated.
To obtain a complete image, the sample is typically scanned. During scanning, the aperture must be maintained at a constant distance from the sample. U.S. Pat. No. 5,354,985 to Quate, which is incorporated herein by reference in its entirety, describes an NSOM in which an optical waveguide is formed along the longitudinal axis of a cantilever. During scanning, the cantilever is positioned parallel to the surface of the sample. A tip is formed near the free end of the cantilever, and the small aperture is formed at the apex of the tip by a focused ion beam process. The cantilever approaches the sample until the apex of the tip is located extremely close to the sample surface. Optical radiation is introduced into the waveguide and transmitted to the tip, where the radiation exits through the aperture. The tip-sample separation is held constant by operating the cantilever in the manner of an atomic force microscope operating in the non-contact or attractive mode. The non-contact or attractive mode is described in numerous sources, including for example Y. Martin et al., "Atomic Force Microscope Force Mapping And Profiling On A Sub-Hundred Angstrom Scale", Journ. App. Phys., Vol. 6, pp. 4723-4729, March 1987.
The NSOM can be operated in the transmission mode, where the sample is analyzed by detecting the light which passes through the sample, or the reflection mode, where the sample is analyzed by detecting the light reflected from the sample.
Another instrument which requires a very small aperture is a scanning ion conductance microscope. In a scanning ion conductance microscope, a very small aperture is formed in a body, typically a pipette, and the body is immersed in an electrolytic bath. An ionic current flows from an electrode located on one side of the body, through the aperture, to one of two electrodes located on the other side of the body in the electrolytic bath.
The resolution obtainable in an NSOM or SICM varies inversely with the width of the aperture and the thickness of the material in which it is formed. In an NSOM, for example, the width of the aperture is normally from 1/10 to 1/20 of the wavelength of the light being used, or in the range of 15-100 nm. Moreover, for commercial instruments the aperture width and thickness should also be predictable and should be repeatable from one manufacturing batch to another.